Photoelectrophoretic imaging process using photoconductive electrode which alters spectral response

ABSTRACT

A photoelectrophoretic imaging system wherein an imaging suspension made up of electrically photosensitive particles dispersed in a carrier liquid are exposed to imagewise radiation and an electrical field causing particle migration in image configuration. The spectral response of the imaging suspension is altered by utilizing a photoconductive electrode.

United States Patent Forest et a1.

1 1 Aug. 26, 1975 PHOTOELECTROPHORETIC IMAGING PROCESS USING PHOTOCONDUCTIVE ELECTRODE WHICH ALTERS SPECTRAL RESPONSE Inventors: Edward Forest, Rochester; Paul C.

Swanton, Webster, both of N,Y.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Oct. 8, 1974 Appl. No: 513,182

Related 0.5. Application Data Continuation of Ser, No 212,704, Dec. 27, 1971, abandoned, which is a continuation of Scr, No. 30,704, April 22, 1970, abandoned, which is a continuatiomin-part of Ser. No. 521,059, Jan, 17, 1966, abandoned.

u.s. Cl 96/11; 96/1 PE; 96/].3 G03q 13/22 Field of Search 96/1 PE, 1.2, 1.3

References Cited UNlTED STATES PATENTS MoncricfflYeates... 96/].3 Moncricff-Yeatcs 96/13 Kaprelian i 4 96/12 Ricker 117/37 LE Gundlach 96/1 R Lehmann 1l7/l7.5 Tulagin et a1, .4 96/1 PE Tulagin 96/1 PE X Wells et a1. 96/1 PE X Primary Examiner-Roland E. Martin, Jr.

ABSTRACT 3 Claims, 2 Drawing Figures PATENTEDAUBZBIBTB 3,901,701

Dmux i DENYSITY E F 2 EXPOSURE-b PHOTOELECTROPHORETIC IMAGING PROCESS USING PHOTOCONDUCTIVE ELECTRODE WHICH ALTERS SPECTRAL RESPONSE CROSS-REFERENCE TO RELATED CASES This Application is a continuation of prior copending Application Ser. No. 212,704, filed Dec. 27, 1971, now abandoned, which is a continuation of prior copending Application Ser. No. 30,704, filed Apr. 22, 1970, now abandoned, which was a continuation impart Applica tion of prior copending Application Ser. No. 521,059, filed Jan. 17, 1966, now abandoned.

BACKGROUND OF THE INVENTION This invention relates in general to imaging systems and, more specifically, to an improved photoelectrophoretic imaging system.

There has been recently developed an electrophoretic imaging system capable of producing either monochromatic or polychromatic images which utilize electrically photosensitive particles. This process is described in detail and claimed in US. Pat. Nos. 3,384,565 to V. Tulagin et a1. and 3,384,566 to H. E. Clark, both issued May 21, 1968. In this imaging sys tem, colored light absorbing particles are suspended in a non-conductive liquid carrier. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electri cally photosensitive and which apparently undergo a net change in charge polarity upon exposure to activat ing electromagnetic radiation, through interaction with one of the electrodes. In a monochromatic system, particles of a single color may be used, producing a colored image equivalent to a conventional black-andwhite photograph. In a polychromatic system, the im' ages may be produced in natural color because mix tures of particles of two or more different colors which are each sensitive only to light ofa specific wave-length or narrow range of wavelengths are used.

In a monochromatic system, black pigments are generally preferred so that a black-on-white image is produced. However, in many instances images in color other than black are preferred, e.g., in poster or sign making. Several different pigments which are capable of producing good images are disclosed in the above noted patents. However, often a pigment which has desirable color characteristics is not sufficiently photosensitive and requires excessive exposure in the image forming step. Some pigments do not produce images of desirable contrast or density. Also, in polychrome imaging where cyan, yellow and magenta particles are used often, one pigment may be substantially more or less responsive than the others rendering faithful color reproduction difficult if not impossible. There is, therefore, a continuing need for methods of varying the im' aging characteristics of these otherwise desirable pigmentsv SUMMARY OF THE INVENTION It is, therefore. an object of this invention to provide a method of photoelectrophoretic imaging which over comes the above-noted disadvantages.

It is another object of this invention to provide a method of varying the effective photosensitivity of pigments in monochromatic and polychromatic photoelectrophoretic imaging systems.

It is another object of this invention to provide a method of varying the density and contrast characteristics of photoelectrophoretic images.

It is still another object of this invention to provide a photoelectrophoretic imaging method capable of utilizing pigments of heretofore insufficient photosensitivity.

It is still another object of this invention to provide a photoelectrophoretic imaging system capable of utilizing a greater variety of pigments than was heretofore thought possible.

The foregoing objects and others are accomplished in accordance with this invention by providing a thin photoconductive layer on an electrode in a photoelectrophoretic imaging system. It has been found that by proper selection of the photoconductor on the injecting electrode color balance, density and contrast characteristics of the resulting image and the effective photosensitivity ofthe image forming material may be varied. The objects and advantages of this improved photoelectrophoretic imaging process will become more apparent upon consideration of the following detailed disclosure of the invention drawings wherein:

FIG, I shows a side view of a simple exemplary system for carrying out the process of this invention; and,

FIG. 2 shows a diagram of a plot of image density against exposure for the imaging process of this invention.

Referring now to FIG. I, there is seen a transparent electrode generally designated I which, in this exem plary instance, is made up ofa layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode will hereinafter be referred to as the injecting electrode". On the surface of injecting electrode 1 is a photoconductive layer 13. Where layer 13 is very thin, it may be applied to the surface of injecting electrode 1 by physically rubbing thereagainst. If it is desired to use a thicker layer 13, any commercial binding agent, such as a thermoplastic resin, may be used to hold it against surface 3 and pre vent dispersion in the carrier liquid applied thereover.

Over layer I3 is coated a thin layer 4 of finely divided photosensitive particles dispersed in an insulating liquid carrier. The term photosensitive for the purposes of this application, refers to the properties of a particle which once attracted to the photoconductive injecting electrode will migrate away from it under the influence of an applied electric field when it is exposed to electromagnetic radiation to which it is sensitive. For a detailed theoretical explanation of the apparent mecha nism of operation of the imaging process, see the above-mentioned US. Pat. Nos. 3,384,565 and 3,384,566 and 3,383,993 issued May 31, 1968 to Yeh, the disclosures ofwhich are incorporated herein by reference. Adjacent to the liquid suspension 4 is a second electrode 5, hereinafter called the blocking electrode" which is connected to one side of a potential source 6 through a switch 7. The opposite side of potential source 6 is connected to the injecting electrode 1 so that when switch 7 is closed, an electric field is applied across the liquid suspension 4 and layer 13 between electrodes 1 and 5. An image projector made up of a light source 8, a transparency 9 and a lens is provided to expose the dispersion 4 to a light image of the original transparency 9 to be reproduced. Electrode Sis made in the form of a roller having a conduc tive central core ll connected through switch 7 to a potential source 6. The core is covered with a layer of a blocking electrode material [2, which may be for example Tedlar or other dielectric material. The pigment suspension is exposed to the image to be reproduced while a potential is applied across the blocking and injecting electrodes by closing switch 7. Roller S is caused to roll across the top surface of injecting electrode l with switch 7 closed during the period of image exposure. This light exposure causes exposed pigment particles originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of the blocking electrode leaving behind a pigment image on the surface of layer 13 which is a duplicate of the original transparency 9. Particles adhering to the surface of blocking electrode may be cleaned therefrom and the exposure steps repeated. if desired. The additional steps of cleaning the blocking electrode and repeating the exposure steps have been found to further increase color purity and color balance. After exposure, the relatively volatile carrier liquid evaporates off. leaving be hind the pigment image. The image may be transferred to a sheet of receiving material. such as paper, by any convenient means.

Layer 13 as described above may comprise any photoconductive material which is capable of modifying the characteristics of the pigment suspension and the images produced thereby.

One imaging characteristic of a pigment suspension which may be varied by the application of a photoconductive composition to the injecting electrode is indir cated in FIG. 2. The curve in FIG. 2 is a plot of image density against exposure time. Maximum and minimum density is indicated as D/max. and D/min., respectively. The slope of the curve is indicative of image contrast or gamma. Photographic speed is measured along the exposure axis to an arbitrary point at the toe of the curve, represented by the dashed line in the diagram. When properly chosen, the photoconductor on the in jecting electrode may modify D/max. and D/min, gamma, photographic speed, and the spectral sensitivity. A photoconductor which is more resistive than the pigment in the imaging suspension will increase D/max. greatly and also affect D/min. to some extent. A more conductive composition will tend to decrease D/max. and D/min. A more highly responsive composition will increase the photographic speed while a relatively insensitive composition will decrease the photographic speed of the imaging pigment. The photoconductor will generally increase the sensitivity of the imaging pigment in the spectral region in which the photoconduc tor absorbs. That is. for example, a magenta photocon ductor which absorbs primarily in the green region of the spectrum will increase the green sensitivity of the imaging pigment.

For full natural color imaging cyan. magenta and yel low particles are used each being sensitive to light in a specific range of wavelengths. For subtractive fullcolor imaging. cyan particles are used which absorb and are primarily responsive to red light; yellow particles which absorb blue light and respond primarily to blue light. and magenta particles responsive mainly to green light are used as described in the above patents.

Use ofa photoconductive layer on the injecting electrode allows the color response of the imaging system to be modified by increasing the response or decreasing the response of a selected pigment. For example. if the images produced on the injecting electrode have too much magenta pigment remaining which gives the whole image a reddish coat, a photoconductor which absorbs green light is used to increase the response of the magenta pigment.

The coating composition on the injecting electrode may comprise any suitable photoconductor composition. The choice of the composition for a particular use will depend upon the the effect desired and the spectral and electrical characteristics of the imaging pigment. For monochrome imaging. photoconductive layers having a more panchromatic response may be preferred. For polychrome imaging, photoconductive layers having a limited spectral response are preferred. Any suitable photoconductors may be used. Typical photoconductors include: inorganic materials such as selenium. zinc oxide. zinc sulfide. cadmium sulfide, cadmium sulfoselenide and mixtures thereof.

Typical organic photoconductors include materials such as polyvinyl carbazole, alone or sensitized with trinitrofluorenone. Preferred materials for use in polychrome imaging include highly pigmented materials having a relatively narrow photosensitive response. Typical materials include: Algol Yellow GC, l,2,5,6- di(C.C'-diphenyl)-thiazole-anthraquinone, Cl. No. 67300, available from General Dye Stuffs; N-2"- pyrid yl-8. l 3-dioxodinaphtho-( l,22',3')-furan-6- carboxamide. prepared as described in copending Application Ser. No. 42!,28l. filed Dec. 28. 1964; Calcium Litho Red. the calcium lake of l-(2- azonaphthalene-l '-sulfonic acidblnaphthol. C.l. NO. I5630, available from Collway Colors; Cyan Blue GTNF, the beta form of copper phthalocyanine, C.l. No. 74160, available from Collway Colors; Diane Blue. 3,3'-methoxy-4.4-diphenyl-bis-( l "-azo2"-hydroxy- 3"-naphthanilide), C.l. No. 21 [80, available from Harmon Colors; 2,4di-( l '-anthraquinonyl-amino)-6-( 1"- pyrcnylytriazine, prepared as described in copending application. Ser. No. 445,l79. filed Apr. 2. 1965; Duol Carmine. the calcium lake of l-(4'- methylazobenzene)-2'-sulfonic acid)-2-hydroxy-3- naphthoic acid. (.l. No. 15850, available from E. l. du- Pont de Nemours. lnc.; lndanthrane Brilliant Orange RK. 4,l0-dibromo-6,] 2-anthanthrone, Cl. NO. 59300, available from General Dye Stuffs; lndofast Brilliant Scarlet Toner. 3.4.9,]0-bislN.N (p-methoxyphenyl)- imidol-perylene, C]. No. 7! 140. available from Harmon Colors; N-3"( l ",2",4"-triazole)-8. l 3-dioxodinaphtho-(2,l-b;2', 3-d)-furan-6-carboxamide; lndofast Violet Lake, dichloro-9,lti-isoviolanthrone, Cl. No. 60010, available from Harmon Colors; lndofast Yellow Toner. flavanthronc, C.|. No. 70600, available from Harmon Colors; 2-(4'-carboxyphenylazo)4-iso propoxy l naphthol; l-cyano-2,3 3-mitro phthaloyl7.8benzopyrrocoline; Locarno Red X4686, L(4'methyl-5'-chlorobenzene-2-sulfonic acidl-Z- hydroxy-3-naphthoic acid. C.l. No. lSXfiS, available from American Cyanamide; Methyl Violet. a phosphotungstomolybdic lake of 4-(N.N'.N'- trimethylanilino)methylene-N'.N'-dimcthylanilinium chloride. C.l. No. 42535, available from Collway Colors; Naphthol Red 3. l-(T-methoxy-Y- nitrophenylazoJ-Z-hydroxy-3"-nitro B-naphthanilide.

Cl. No. l2355, available from Collway Colors; Monolite Fast Blue GS. a mixture of the alpha and beta forms of metal-free phthaloctanine. available from the Arnold Hoffman Co.; Permagen Red L Toner 51-500, l-(4'-methyl-5'-chloroazobenzene-2'-sulfonic acid)2- hydroxy-B-naphthoic acid, Cl. No. [5865, available from Collway Colors; Vulcan Fast Red BBE Toner 35-2201. 3,3'dimethoxy-4.4'-biphenyl-bis( l"-phenyl- 3"-methyl-4"-azo 2"pyrazoline-5"-one). Cl. No. 2l200. available from Collway Colors; Quindo Magenta RV-6803, a substituted quinacridone. available from Harmon Colors; Watchung Red B, l (4'-methyl- 5-chloroazobenzene-2'-sulfonic acid )-2-hydroxy-3- naphthoic acid. C.l. No. l5865. available from E. 17 du Pont de Nemours; and 8.13dioxodinaphtho-( 1.2- b;2',3'-d)-furan-6-carbox-4"-methoxyanilide.

The coating of photosensitive material on the injecting electrode may have a thickness of up to 1 micron. The thicker layers may include a binder material, such as polyvinyl polystyrene. epoxy resins, phenolic resins, silicone resins, urethanes; and mixtures and c0poly mers thereof. Optimum results. however, have been obtained with very thin layers. Layers having an optical density of about 005 have shown to give excellent results and are. therefore. preferred. Layers of this thick ness do not require a binder. Merely rubbing the coating composition against the injecting electrode surface will produce a layer of this thickness which will not be disturbed by the suspension of photosensitive pigments and carrier liquid which is applied thereover.

Imaging suspensions useful in this process include those listed in U.S. Pat. No. 3,384,488 issued May 2] 1968 to V. Tulagin and L. Carriera, the disclosure of which is incorporated herein by reference.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to modifying photoelectrophoretic images by means of photoconductor layers on the injecting electrode. The parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the photoelectrophoretic imaging methods of this invention.

All of the following examples are carried out in an apparatus of the general type illustrated in the figure. The NESA glass surface is connected in series with a switch, a potential source. and a conductive center of a roller having a coating of Baryta paper on its surface. The roller is approximately 2 /2 inches in diameter and is moved across the plate surface at about 1.5 centimeters per second. The plate employed is roughly 3 inches square and is ex osed by light intensity of about 8000 foot candles as measured on the uncoated NESA glass surface. Unless otherwise indicated, 7 percent by weight of the indicated pigment in each example is suspended in Sohio Odorless Solvent 3440 a kerosene fraction available from Standard Oil of Ohio. The magnitude of the applied potential is 2500 volts. Exposure is made with a 3200K lamp. The pigments are solvent extracted and milled to reduce their particle size to preferably one micron or less.

EXAMPLE I PRIOR ART A suspension of a yellow pigment. 8J3- dioxodinaphtho( 2, l -b;2 ',3-d )-furan-6 carbox-4"- methoxyanilide prepared as described in copending application, Ser. No. 421.377. filed Dec. 28, 1964. in Sohio Odorless Solvent 3440. is prepared and coated on the NESA glass surface. A negative potential of about 2.500 volts is imposed on the roller electrode. The image is projected on the suspension while the roller is passed across the injecting electrode surface. An image corresponding to the original is produced on the NESA surface. Here. D/max. is quite low. about 0.05.

EXAMPLE II A pigment suspension is prepared as in Example I above. Watchung Red B, l'(4'-methyl-5'- chloroazobenzene 2'-sulf0nic acid)-2-hydroxy-3- naphthoic acid, C.l. No. l5865, available from E. l. du Pont de Nemours and Co. is rubbed on the NESA injecting electrode surface until the layer has an optical density of about 0.05. Watchung Red B is more resistive than the yellow pigment used in the suspension. The yellow pigment suspension is then coated onto the injecting electrode and a negative potential is imposed on the roller electrode. The suspension is imaged as above. producing an image on the injecting electrode surface corresponding to the original. Image quality is noticeably improved. The D/max. is increased to about 05. About I 1 steps on the neutral density step wedge filter are reproduced.

EXAMPLE III A yellow pigment suspension is prepared as in Example l above. The surface of the NESA injecting elec trode is rubbed with Monolite Fast Blue GS, a mixture of alpha and beta forms of metal-free phthalocyanine. available from Arnold Hoffman Co.. a division of lCl, Ltd. until an optical density of about 0.05 is attained. The yellow pigment suspension is coated onto the injecting electrode and a negative potential is imposed on the roller electrode. The plate is exposed to a black and white transparency which includes a conventional neutral density step wedge filter. While the density of the image produced is still very low, white light sensitivity is greater here since the red sensitivity of the suspension is increased.

EXAMPLE IV PRIOR ART A suspension comprising about 7 parts Watchung Red B and about parts Sohio Odorless Solvent 3440 is prepared. The suspension is coated on the NESA injecting electrode and a negative potential is imposed on the blocking electrode. The suspension is exposed through a transparency which includes color patches and a neutral density step wedge filter. An image is produced on the NESA surface corresponding to the original. The image has a D/max. of about 1.5. The suspension is primarily sensitive to green light. with some blue sensitivity. The suspension is relatively insensitive to red light.

EXAMPLE V The coating and imaging steps are repeated as in Example lV above. except that the NESA surface is rubbed with Monolite Fast Blue OS to an optical density of about 0.05 before the pigment suspension is applied thereto. Monolite Fast Blue GS is more photosensitive in red light than is the magenta pigment used in the suspension. The resulting image if found to have a lower D/max., about 0.5. No effect on D/min. is observed with green or blue exposures. Red sensitivity of the suspension is increased.

EXAMPLE VI The coating and imaging steps are repeated as in Example IV above except that the NESA surface is rubbed with the yellow pigment described in Example I above to an optical density of about 0.03 before the pigment suspension is applied thereto. This yellow pigment is less resistive than the magenta pigment used in the suspension. There, the D/max. is decreased from about l.5 observed on clean NESA to about 0.5. A slight increase in blue sensitivity is also observed.

EXAMPLE VII PRIOR ART A suspension is prepared comprising about parts Calcoloid Gray 2 pigment available from American Cy anamid, prepared as described in US. Pat. No. 2,456,589, in about 55 parts Sohio Odorless Solvent 3440. The roller electrode is maintained at a positive potential of about 2,500 volts. The suspension is exposed to an image through a black-and-white transparency which includes a neutral density step wedge area. After the imaging steps, an image is observed on the NESA glass corresponding to the original. The image is of good quality and about 5 density steps are descern ible.

EXAMPLE VIII A suspension is prepared and imaged as in Example VII above, except that the NESA injecting electrode is coated with Monolite Fast Blue GS to an optical density of about 0.l6 before the suspension is applied thereto. The resulting image is of higher quality and about 7 density steps are discernible.

EXAMPLE IX The suspension preparing and imaging steps are repeated as in Example VII above, except that the NESA injecting electrode is coated with Monolite Fast Blue GS to an optical density of about 0.20 before the pig ment suspension is applied thereto. The resulting image is of excellent quality and about l 1 density steps are discernible. A reduction of D/min. and an increase in range and speed has resulted from the coating on the injecting electrode surface. The speed of the suspen sion is about 64 times greater than in Example VII where the injecting electrode was not coated.

EXAMPLE X PRIOR ART An imaging suspension comprising about 3 parts by weight of a cyan pigment, Monolitc Fast Blue GS the alpha form of metal-free phthalocyaninc, about 3 parts by weight ofa yellow pigment N-2"-pyrid vl-8,1 3-diox odinaphtho-(ll-b; 2',3 d)furan6-carboxamide and about 5 parts of a magenta pigment Bonadur Red B, an insolublilizcd azo dye C1. No. 15865 with hydrogen substituted for the sodium substituent is dispersed in about 90 parts Sohio Odorless Solvent 3440 is coated on the NESA glass surface. The suspension is exposed to a full color Kodachrome" transparency. The light intensity is about 600 foot candles as measured on the uncoated NESA glass surface. Two rollers traverses are made with the roller being held at a potential of about till a negative 3,000 volts with respect to the NESA surface. The full color positive image remaining on the NESA is transferred to paper. The image is found to be of fair quality with a magenta D/min, and poor green and yellow rendition due to the presence of unwanted magenta particles.

EXAMPLE XI The experiment of Example X is repeated except that the NESA electrode is first coated as in Example II. Inspection of the image produced shows that the magenta D/min. has been reduced and the green and yellow reproduction is improved.

EXAMPLE Xll PRIOR ART The experiment of Example X is repeated except that the magenta pigment in the suspension is Watchung Red B. Examination of the formed image again shows a magenta D/min. with poor green and yellow reproduction.

Although specific components and proportions have been stated in the above description of preferred embodiments of the invention, other suitable materials. as listed above, may be used with similar results. In addi tion, other materials may be added to the imaging suspension and the pigment layer to synergize, enhance, or otherwise modify their properties. For example, the photoconductive layers may be overcoated with a transparent resin to increase their durability.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

l. A photoelectrophoretic imaging method compris ing a. providing a pair of electrodes one of which is conductive and has a photoconductive overcoating, wherein said conductive electrode and said photoconductive overcoating are at least partially transparent to activating electromagnetic radiation and wherein said photoconductive overcoating comprises l( 4 '-mcthoxy-5 -chIoroazobenzene-2'- sulfonic acid )Q-hydroxy-3-naphthoic acid. C]. No. 15865;

b. providing a layer of an imaging suspension comprising cyan. magenta, and yellow particles in an electrically insulating carrier liquid on said photoconductive overcoating, each said particle comprising an electrically photosensitive pigment;

c. subjecting said suspension layer to an applied electrical field between said electrodes; and

d. exposing said suspension through said photocon ductive overcoating to an imagewisc pattern of visible light corresponding to a full color original image whereby there is formed a full color image.

2. The method as defined in claim 1 wherein said photoconductivc ovcrcoating has a thickness of up to about one micron.

3. The method as defined in claim I wherein said photoconductive ovcrcoating has an optical density of about 0.05 

1. A PHOTOELECTROPHORETIC IMAGING METHOD COMPRISING A. PROVIDING A PAIR OF ELECTRODES ONE OF WHICH IS CONDUCTIVE AND HAS A PHOCONDUCTIVE OVERCOATING, WHEREIN SAID CONDUCTIVE ELECTRODE AND SAID PHOTOCONDUCTIVE OVERCOATING ARE AT LEAST PARTIALLY TRANSPARET TO ACTIVATING ELECTROMAGNETIC RADIATION AND WHEREIN SAID PHOTOCONDUCTIVE OVERCOATING COMPRISES 1-(4,-METHOXY-5,-CHLOROAZOBENZENE-2,-SULFONIC ACID)-2-HYXDROXY-3-NAPHTHOIC ACID, C.I. NO. 15865, B. PROVIDING A LAYER OF AN IMAGING SUSPENSION COMPRISING CYAN, MAGNETA, AND YELLOW PARTICLES IN AN ELECTRICALLY INSULATING CARRIER LIQUID ON SAID PHOTOCONDUCTIVVE OVERCOATING, EACH SAID PARTICLE COMPRISING AN ELECTRICALLY PHOTOSENSITIVE PIGMENT, C. SUBJECTING SAID SUSPENSION LAYER TO AN APPLIED ELECTRICAL FIELD BETWEEN SAID SELECTRODES, AND D. EXPOSING SAID SUSPENSION THROUGH SAID PHOTOCONDUCTIVE OVERCATING TO AN IMAGEWISE PATTERN OF VISIBLE LIGHT CORRESPONDING TO A FULL COLOR ORIGINAL IMAGE WHEREBY THERE IS FORMED A FULL COLOR IMAGE.
 2. The method as defined in claim 1 wherein said photoconductive overcoating has a thickness of up to about one micron.
 3. The method as defined in claim 1 wherein said photoconductive overcoating has an optical density of about 0.05. 